(a) Field of the Invention
The present invention relates to a method and an apparatus for setting a quiet window in a passive optical network system.
(b) Description of the Related Art
In accordance with the development of optical communication technology and a rapid increase in the Internet service demand, a study on ultra-high speed large capacity optical subscriber network technology has been actively conducted. A passive optical network (PON) has a time division multiplexing (TDM) structure in which a plurality of optical network units (ONUs) are connected to one optical line terminal (OLT) through a passive element in a point-to-multipoint scheme. In the PON, an optical network terminal (ONT) registration process is required in order for the OLT to manage the ONUs connected thereto and allocate bands for an uplink traffic. To this end, the OLT allocates ONU-IDs, which are identifiers, to the respective ONUs. The registration process of the ONU includes a serial number acquisition process and a ranging process.
In the serial number acquisition process, when the OLT transmits a specific uplink bandwidth map to all the ONUs in a broadcast scheme, the ONUs transfer serial numbers together with random delay values used by the ONUs to the OLT through physical layer operations, administration and maintenance (PLOAM) messages after a random time delay. The random time delay prevents the PLOAM messages transmitted by a plurality of ONUs from being collided with each other. In this case, the transmitted specific uplink bandwidth map may be called a serial number request. When the OLT does not acquire the serial numbers of the ONUs due to collision generated between the PLOAM messages transmitted by the plurality of ONUs, the same operation is repeated in the next serial number acquisition process.
The OLT performs the ranging process individually on ONUs of which serial numbers are normally acquired, transmits an equalization delay value for temporally aligning the ONUs for the purpose of TDM transmission, and finishes the registration process.
Meanwhile, during the serial number acquisition process, a section in which the OLT transmits empty band allocation map information, such that uplink data of registered ONUs are not transmitted is called a quiet window. For a time corresponding to the quiet window, the ONUs that are registered and are served do not transmit traffics, but buffer the traffics. The OLT periodically generates a quiet window section in order to recognize existence of newly registered ONUs, and this period is not standardized in a standard document, but may be generally set to several milliseconds (ms) to several seconds (s).
A size of the quiet window section is associated with a distance at which the ONUs are positioned, and according to a standard specification, it has been recommended to use 250 μs in the case in which a differential distance (a distance difference between an ONU positioned at a position that is closest to the OLT and an ONU positioned at a position that is farthest from the OLT) is, for example, 20 km, and use 450 μs in the case in which the differential distance is 40 km. Therefore, when positions of the ONUs may be recognized, a time of the quiet window section may be reduced, and thus, buffering times of the ONUs that are being served may be shortened, thereby making it possible to improve quality of service (QoS).
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.